There is known a head-mounted display (HMD) mounted on the head of a user to view a video as described, for example, in PTL 1 to 3. There is an HMD including a plurality of light-emitting units, such as light-emitting diodes (LEDs), that emit light at luminance according to drive current, and the posture of the HMD can be specified based on an image of the plurality of light-emitting units taken by a camera.